Heat pump
A system that moves heat rather than creating it, and can run in both directions: taking heat out of a building in cooling and drawing heat in from outside air, ground, or water in heating. The same hardware provides both functions, which is why one fault can affect heating and cooling together.
Also called ASHP, air source heat pump, reverse cycle
What is a heat pump?
A heat pump is a refrigeration system that moves heat instead of burning fuel to make it, and reverses its refrigerant flow so the same equipment can both heat and cool a building.
Heat pump: how it works
Refrigerant boiling at a low temperature absorbs heat from a cold source, the compressor raises its pressure so it condenses at a higher temperature, and that heat is released where it is wanted. Because the energy is moved rather than generated, output per unit of electricity is higher than direct electric heating, and it falls as the gap between source and target temperature widens.
Where you meet the term
- On a grant or incentive application, where the rated efficiency and the heating capacity at a stated outdoor temperature decide eligibility.
- In a complaint about weak heating during a cold snap, where reduced output can be correct operation rather than a fault.
- In a code definition, because a heating fault and a cooling fault can involve the same hardware and therefore share codes.
How to check it
- Confirm the mode the system is actually in. A heat pump paused for defrost has stopped heating on purpose. Check the display and wait for the cycle to finish before treating it as a fault. Owner check
- Compare output against outdoor temperature. Rated capacity is published at specific outdoor conditions. Reduced output at an outdoor temperature well below the rating point is expected behaviour. Technician
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| Heat moved per unit of electricity | Expressed as COP | A COP of 3 means three units of heat delivered for one unit of electricity consumed |
| Seasonal figure in Europe | SCOP | Averaged across a defined heating season and climate profile |
| Seasonal figure in the United States | HSPF2 | Not directly comparable with SCOP because the test procedures differ |
United States and United Kingdom usage
- United States
- A ducted or ductless air-to-air system paired with electric auxiliary heat, rated with SEER2 and HSPF2.
- United Kingdom
- An air-to-water system replacing a boiler and feeding radiators or underfloor loops, rated with SCOP.
Heat pump: what it is not
- A heat pump does not create heat from electricity the way a resistance heater does, which is why its efficiency figures exceed one.
- Cold weather does not stop a heat pump working. Output falls as outdoor temperature drops, and the balance point is where that fall starts to matter.
- Air conditioning and heat pump hardware are close relatives. The reversing valve, not the compressor, is what makes one of them reversible.
Heat pump questions
Does a heat pump work below freezing?
Yes. Air below freezing still contains heat, and modern units are rated to extract it. The amount delivered and the electricity required change with temperature, which is why the balance point matters.
Why does my heat pump blow cool air sometimes in heating?
During a defrost cycle the system briefly runs in reverse to clear frost from the outdoor coil, and indoor air can feel cool for a few minutes. The cycle ends on its own.
Is an air conditioner a heat pump?
It uses the same refrigeration cycle but runs in one direction only. A heat pump adds a reversing valve and the controls to use it, which is what allows it to heat.
Where this definition comes from
Written from government guidance, manufacturer support article material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Heat Pump SystemsConsumer-facing explanation of air source, ductless, and geothermal heat pump operation and efficiency metrics.
- Why Is My Mini-Split Not Working in Heat Mode?Trane ductless heating checks covering controls, airflow, outdoor clearance, and service conditions.